Thanks for all your replies and helpful advice.
Thought I would post an after the event update for anyone that is interested.
All went well and would you believe it never rained a drop for the six days of the ride. That must be some kind of record!
We rode from Salisbury to Chester basically up Sustrans route 42, known in parts as the Mercian way. The whole ride was fantastic, challenging enough for a beginner to feel that I had earned my pint at the end of the day but nothing too radical. Plenty of tea shops and cake as well.
The rear end was ok, I think I the few training rides I did really paid off probably more in bum comfort that fitness.
I now understand that weight is everything when touring and got my load down to 7 kg for the journey. I guess some of you would still think that's a lot!!
The best news is that I'm now hooked on cycling, I even brushed off an old mountain bike in the garage and have been doing lots of local off road rides, not real mountains, just bridle ways and back lanes but I've been to places locally that I never new existed.
Thanks again for all your interest and advice, I think you may well have contributed to my enjoyment of the ride and helped me on my way to becoming a cyclist.